Android HandlerThread使用及原理分析

子线程的消息循环

很明显的一点就是，我们要在子线程中调用Looper.prepare() 为一个线程开启一个消息循环，默认情况下Android中新诞生的线程是没有开启消息循环的。（主线程除外，主线程系统会自动为其创建Looper对象，开启消息循环。） Looper对象通过MessageQueue来存放消息和事件。一个线程只能有一个Looper，对应一个MessageQueue。 然后通过Looper.loop() 让Looper开始工作，从消息队列里取消息，处理消息。

public class MainActivity extends AppCompatActivity {

private Handler handler1 ;
private Handler handler2 ;

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
new MyThread1().start();
new MyThread2().start();
}

class MyThread1 extends Thread {

@Override
public void run() {
super.run();

Looper.prepare();

handler1 = new Handler(){
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
System.out.println( "threadName--" + Thread.currentThread().getName() + "messageWhat-"+ msg.what );
}
};

try {
sleep( 3000 );
} catch (InterruptedException e) {
e.printStackTrace();
}

handler2.sendEmptyMessage( 2 ) ;

Looper.loop();
}
}

class MyThread2 extends Thread {
@Override
public void run() {
super.run();
Looper.prepare();

handler2 = new Handler(){
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
System.out.println( "threadName--" + Thread.currentThread().getName() + "messageWhat-"+ msg.what );
}
};

try {
sleep( 4000 );
} catch (InterruptedException e) {
e.printStackTrace();
}

handler1.sendEmptyMessage( 5 ) ;

Looper.loop();
}
}
}

然而这一切都可以用HandlerThread类来帮我们做这些逻辑操作。

HandlerThread

HandlerThread本质上就是一个普通Thread,只不过内部建立了Looper.

HandlerThread的常规用法

创建一个HandlerThread

mThread = new HandlerThread("handler_thread");`

启动一个HandlerThread

mThread.start();

退出循环

Looper是通过调用loop方法驱动着消息循环的进行: 从MessageQueue中阻塞式地取出一个消息，然后让Handler处理该消息，周而复始，loop方法是个死循环方法。

那如何终止消息循环呢？我们可以调用Looper的quit方法或quitSafely方法，二者稍有不同。

public void quit() {
mQueue.quit(false);
}

public void quitSafely() {
mQueue.quit(true);
}

相同点：

将不在接受新的事件加入消息队列。

不同点

当我们调用Looper的quit方法时，实际上执行了MessageQueue中的removeAllMessagesLocked方法，该方法的作用是把MessageQueue消息池中所有的消息全部清空，无论是延迟消息（延迟消息是指通过sendMessageDelayed或通过postDelayed等方法发送的需要延迟执行的消息）还是非延迟消息。

当我们调用Looper的quitSafely方法时，实际上执行了MessageQueue中的removeAllFutureMessagesLocked方法，通过名字就可以看出，该方法只会清空MessageQueue消息池中所有的延迟消息，并将消息池中所有的非延迟消息派发出去让Handler去处理，quitSafely相比于quit方法安全之处在于清空消息之前会派发所有的非延迟消息。

无论是调用了quit方法还是quitSafely方法只会，Looper就不再接收新的消息。即在调用了Looper的quit或quitSafely方法之后，消息循环就终结了，这时候再通过Handler调用sendMessage或post等方法发送消息时均返回false，表示消息没有成功放入消息队列MessageQueue中，因为消息队列已经退出了。

需要注意的是Looper的quit方法从API Level 1就存在了，但是Looper的quitSafely方法从API Level 18才添加进来。

小例子

import android.os.Bundle;
import android.os.Handler;
import android.os.HandlerThread;
import android.os.Message;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;

public class MainActivity extends AppCompatActivity {

private HandlerThread myHandlerThread ;
private Handler handler ;

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);

//创建一个线程,线程名字：handler-thread
myHandlerThread = new HandlerThread( "handler-thread") ;
//开启一个线程
myHandlerThread.start();
//在这个线程中创建一个handler对象
handler = new Handler( myHandlerThread.getLooper() ){
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
//这个方法是运行在 handler-thread 线程中的 ，可以执行耗时操作
Log.d( "handler " , "消息： " + msg.what + "  线程： " + Thread.currentThread().getName()  ) ;

}
};

//在主线程给handler发送消息
handler.sendEmptyMessage( 1 ) ;

new Thread(new Runnable() {
@Override
public void run() {
//在子线程给handler发送数据
handler.sendEmptyMessage( 2 ) ;
}
}).start() ;

}

@Override
protected void onDestroy() {
super.onDestroy();

//释放资源
myHandlerThread.quit() ;
}
}

运行效果：

/com.app D/handler: 消息： 1  线程： handler-thread
/com.app D/handler: 消息： 2  线程： handler-thread

HandlerThread源码分析

**
* Handy class for starting a new thread that has a looper. The looper can then be
* used to create handler classes. Note that start() must still be called.
*/
public class HandlerThread extends Thread {
int mPriority;
int mTid = -1;
Looper mLooper;

public HandlerThread(String name) {
super(name);
mPriority = Process.THREAD_PRIORITY_DEFAULT;
}

/**
* Constructs a HandlerThread.
* @param name
* @param priority The priority to run the thread at. The value supplied must be from
* {@link android.os.Process} and not from java.lang.Thread.
*/
public HandlerThread(String name, int priority) {
super(name);
mPriority = priority;
}

/**
* Call back method that can be explicitly overridden if needed to execute some
* setup before Looper loops.
*/
protected void onLooperPrepared() {
}

@Override
public void run() {
mTid = Process.myTid();
Looper.prepare();
synchronized (this) {
mLooper = Looper.myLooper();
notifyAll();
}
Process.setThreadPriority(mPriority);
onLooperPrepared();
Looper.loop();
mTid = -1;
}

/**
* This method returns the Looper associated with this thread. If this thread not been started
* or for any reason is isAlive() returns false, this method will return null. If this thread
* has been started, this method will block until the looper has been initialized.
* @return The looper.
*/
public Looper getLooper() {
if (!isAlive()) {
return null;
}

// If the thread has been started, wait until the looper has been created.
synchronized (this) {
while (isAlive() && mLooper == null) {
try {
wait();
} catch (InterruptedException e) {
}
}
}
return mLooper;
}

/**
* Quits the handler thread's looper.
* <p>
* Causes the handler thread's looper to terminate without processing any
* more messages in the message queue.
* </p><p>
* Any attempt to post messages to the queue after the looper is asked to quit will fail.
* For example, the {@link Handler#sendMessage(Message)} method will return false.
* </p><p class="note">
* Using this method may be unsafe because some messages may not be delivered
* before the looper terminates.  Consider using {@link #quitSafely} instead to ensure
* that all pending work is completed in an orderly manner.
* </p>
*
* @return True if the looper looper has been asked to quit or false if the
* thread had not yet started running.
*
* @see #quitSafely
*/
public boolean quit() {
Looper looper = getLooper();
if (looper != null) {
looper.quit();
return true;
}
return false;
}

/**
* Quits the handler thread's looper safely.
* <p>
* Causes the handler thread's looper to terminate as soon as all remaining messages
* in the message queue that are already due to be delivered have been handled.
* Pending delayed messages with due times in the future will not be delivered.
* </p><p>
* Any attempt to post messages to the queue after the looper is asked to quit will fail.
* For example, the {@link Handler#sendMessage(Message)} method will return false.
* </p><p>
* If the thread has not been started or has finished (that is if
* {@link #getLooper} returns null), then false is returned.
* Otherwise the looper is asked to quit and true is returned.
* </p>
*
* @return True if the looper looper has been asked to quit or false if the
* thread had not yet started running.
*/
public boolean quitSafely() {
Looper looper = getLooper();
if (looper != null) {
looper.quitSafely();
return true;
}
return false;
}

/**
* Returns the identifier of this thread. See Process.myTid().
*/
public int getThreadId() {
return mTid;
}
}

HandlerThread的特点

HandlerThread将loop转到子线程中处理，说白了就是将分担MainLooper的工作量，降低了主线程的压力，使主界面更流畅。

开启一个线程起到多个线程的作用。处理任务是串行执行，按消息发送顺序进行处理。HandlerThread本质是一个线程，在线程内部，代码是串行处理的。

但是由于每一个任务都将以队列的方式逐个被执行到，一旦队列中有某个任务执行时间过长，那么就会导致后续的任务都会被延迟处理。

HandlerThread拥有自己的消息队列，它不会干扰或阻塞UI线程。

对于网络IO操作，HandlerThread并不适合，因为它只有一个线程，还得排队一个一个等着。